As uninterruptible power supply apparatuses, an apparatus called continuous commercial power feeding system has hereto been employed. In this system, in normal operation, the input power is directly supplied to a load while charging a battery through a charging circuit; and, in the event of a power failure, power is supplied from the battery by mechanically switching a relay from commercial power supply to an inverter circuit. Although this system has an advantage of a low cost and a low loss because the specification has now become standard, it suffered problems of being susceptible to the fluctuation of the commercial power supply as the input power is directly supplied to the load and also being difficult to make it smaller and lighter because a large size transformer is required. It also suffers added disadvantage of requiring a switching time in the event of a power failure for mechanically switching a relay to obtain power from the battery. For instance, when a computer is used as a load apparatus, fluctuation of the power supply voltage or suspension of power supply during the period of switching may cause malfunction of the computer, leaving room for improvement.
There was available a continuous commercial power feeding system with auto voltage regulation function in which the continuous commercial power feeding system had been improved by the use of a voltage step-up/step-down circuit to cope with a fluctuation of the commercial power. With this system, it was possible to maintain the supply voltage to the load within a predetermined range by stepping up the output voltage when the supply voltage had dropped and stepping down the output voltage when the supply voltage had increased. This way, it became possible to supply a stable voltage to the load when compared with the aforementioned system; for example the output voltage range could be maintained at 100+/-10 V for an AC input voltage of 100 V. However, in this system, too, perfectly stable supply of voltage to the load was not possible because of the time required for switching for voltage step-up or step-down. Also, there was a risk of accelerating deterioration of the battery due to battery backed-up operation for several seconds during switching for voltage step-up or step-down. Furthermore, problems of difficulty for smaller size and lighter weight design remained to be solved.
As another system other than the continuous commercial power feeding system, a continuous inverter power feeding system has been in actual use. In this system, in the normal condition, power is supplied to a load apparatus by first converting commercial power supply into a DC power followed by reconverting it back to an AC power by an inverter circuit while at the same time charging a battery with a charging circuit. In the event of a power failure, power is supplied to the load apparatus by operating the inverter circuit using the battery as the power supply. In this system, as power is supplied via an inverter circuit, it is always possible to supply a nearly constant voltage within a range of +/-3% to the load even when there is a fluctuation in the commercial power supply. Also, as there is no mechanical switching circuit, it is possible to supply power to the load apparatus without instantaneous interruption even in the event of a power failure. Furthermore, as a large size transformer is not required, it does not suffer the problems of the above-described continuous commercial power feeding system.
However, from the standpoint of power saving, the continuous inverter power feeding system in which an internal circuit was in constant operation consumed more power and was clearly inferior to the two earlier described types of continuous commercial power feeding system.
As an uninterruptible power supply apparatus has been designed and developed with a view to protecting supply of power to connected load apparatus such as a computer and the like, there has only been a notion of improving efficiencies of the AC-to-DC converter or DC-to-AC inverter from the power saving standpoint of the main unit of an uninterruptible power supply apparatus of the continuous inverter power feeding type. As one of the approaches toward this, there has been proposed a power-saving type uninterruptible power supply apparatus in which the voltage at which the efficiency of the load apparatus is the highest and the power dissipation is the lowest is judged by varying the inverter voltage within the allowable input voltage range of the load apparatus, and the inverter output voltage is fixed at the judged voltage during operation.
However, in the conventional uninterruptible power supply apparatus of the continuous inverter power feeding type, almost no consideration has been made on the power saving of the uninterruptible power supply apparatus as a whole or, further, power saving of the total system including the connected load apparatus such as a computer and peripheral devices connected to the load apparatus such as a printer, display, and the like. For this reason, power has been wasted when viewed as a total system, presenting problems to be improved.
The present invention addresses these problems and aims at achieving power saving of an uninterruptible power supply apparatus of the continuous inverter power feeding type as a whole as well as power saving of the total system including the uninterruptible power supply apparatus and connected load apparatus such as a computer and peripheral devices attached to the load apparatus.